This research is dedicated to testing two related biological hypotheses. The first hypothesis is: if the proper stimulation sequence can be found, then it is possible to significantly shorten AV nodal effective refractory period (ERP) and simultaneously lengthen AV (and/or) VA nodal conduction time. Our preliminary work suggests the validity of this hypothesis. The second biological hypothesis is: If sufficient ERP shortening can be induced concomitant with sufficiently increased nodal conduction time then reentrant nodal tachycardias can be induced with the single additional premature stimulus. This hypothesis assumes functional longitudinal dissociation for part of the AV conduction system. To test these hypotheses profiles of ERP and AV conduction time as a function of each preceding cycle length are rapidly measured by an automated system based upon a minicomputer and CRT/keyboard terminal which we have developed. The arrhythmia induction protocol we expect to find will aid in the study of the mechanisms of circus movement tachycardias. The isolated rabbit heart will be used.